The present invention relates to a mobile communication system which provides a transmission power control circuit for controlling transmission power, and more particularly to the mobile communication system which is suitable to a car phone or a portable phone provided with a direct spreading code divisional multi access system.
Conventionally, there have been known various kinds of systems as a cellular system for a car phone or a portable phone system. Those known systems include a Japanese standard system (PCD: RCR STD 27), a North America standard system (TIA IS54), and a European standard systems, these of which utilize a time divisional multi access (TDMA), and another North America standard system (TIA IS95) which utilizes a code divisional multi access (CDMA).
A cellular system that utilizes the code divisional multi access (CDMA) system is arranged so that two or more mobile phones are connected to one base station through carriers of one frequency. This cellular system is required to provide the base station with a channel transmission power control technique for keeping the signal powers from plural mobile phones identical with each other. This ground will be described below. For example, assume that the power received from one mobile phone is ten times so great as the power received from another mobile phone. The former mobile phone gives ten times as great a channel interference as another mobile phone to the power received from another mobile phone. In other words, the former mobile phone brings about the channel interference correspond to that of ten ordinary mobile phones. In a case that the power received from one mobile phone is ten times as great as that received from the other mobile phones, the number of mobile phones or channels to be connected to one base station at a time is reduced by nine, as compared with the case that the base station receives the same power from each of all mobile phones connected thereto.
The cellular system that utilizes the CDMA needs to control the transmission power so that the base station can receive the same power from each of the connected mobile phones. The malfunction of this control disadvantageously leads to greatly reducing the number of channels to be connected as system capacitance.
The control for a transmission power on an up link executed in the North America standard system that utilizes the code divisional multi access will be discussed in detail in the document ITA/EIA/IS-95-A, chapters 6 and 7 issued by TIA, for example. The transmission power of the mobile phone is controlled by an open loop power control or a closed loop power control. In the open loop power control, the mobile phone measures the received power on the down link, estimates a propagation loss based on a difference between the received power and the power sent by the base station, and decide a power to be transmitted by the mobile phone itself based on the propagation loss. ("Estimated Open Loop Output Power" of 6.1.2.3.1 of the document TIA/EIA/IS-95-A issued by TIA and "Open Loop Estimation" of 6.1.2.4.1 of this document). In this open loop power control, the down link has a different frequency from the up link, so that the down transmission loss does not necessarily coincide with the up transmission loss. Hence, only this open loop power control cannot minutely control the transmission power.
Turning to the closed loop power control, the base station measures the received power at a unit of a time slot of 1.25 ms and decide the magnitude of the received power on a reference value. At the succeeding slots of the down link, in a case that the base station decides the received power is greater than the reference value, the base station gives to the mobile phone an indication that the power to be transmitted by the mobile phone is changed by -1 dB. In a case that the base station decided the received power is smaller than the reference value, the base station gives to the mobile phone an indication that the power to be transmitted by the mobile phone is changed by +1 dB. In response to the indication about change of the transmission power from the base station, the mobile phone changes the transmission power at the next slot to the slot when the indication is given. (Refer to "Closed Loop Correction" of 6.1.2.4.2 of the document TIA/EIA/IS-95-A issued by TIA and "Power Control Subchannel" of 7.1.3.1.7 of this document.)
The North American standard system that utilizes the CDMA supports a variable rate speech vocoder (speech encoder). That is, at a normal telecommunication channel (TCM), the bit rate is 9600 bps, while at an interval where no speech takes place (voiceless interval), the bit rate is reduced from a half to a one-eighth for reducing the interference caused for another channel. Concretely, one frame of 20 ms is divided into 16 time slots, each length of which is 1.25 ms. Of these time slots, a half to a one-eighth time slots are selectively transmitted by using a pseudo random variable and the other time slots are not transmitted for implementing the transmission at a variable rate. The base station indicates the change of the transmission power according to the foregoing procedure irrespective of whether or not the transmission is done at the current time slot, while the mobile phone changes the transmission power according to only the indication for change of the transmission power for the time slot where the actual transmission is done.
As is obvious from the foregoing description, if the variation of a transmission loss, that is, fading or shadowing is gradually caused, the open loop power control and the closed loop power control can be used for controlling the power received from the mobile phone to the base station in the range of .+-.1 dB.
As described in the foregoing publication ("Reverse CDMA Channel Signals" of 6.1.3.1 of the document TIA/EIA/IS-95-A issued by the TIA), the up link is 64-ary-quadrature-modulated and asynchronously detected. Then, the resulting signal is subject to the RAKE combine and the antenna diversity combine. The received power is obtained by measuring these combined powers.
In the closed loop power control system used in the North America standard system provided with the CDMA as described above, when the base station measures the power transmitted by the mobile phone at the N-th slot, the base station gives to the mobile phone an indication about change of the transmission power at the (N+2)th slot, and then the mobile phone modifies the transmission power at the (N+3)th slot. That is, the control is delayed by three slots. Assuming that one slot is 1.25 ms, the control is delayed by 3.75 ms.
In this kind of control for the transmission power, if the variation of the transmission path is far slower than the control delay of 3.75 ms, that is, 1/267 Hz, this control is valid. If the variation is faster, this control for the transmission power is not valid. In particular, if a high frequency such as 2 GHz is used, the variation of the transmission path is likely to be faster. This may disable the control.
In the North America standard system that utilizes the CDMA, the control delay (3 slots=3.75 ms) is greater than the control period (1 slot=1.25 ms). Hence, when the transmission path is gradually varying, the oscillation takes place at four times as great a period as the control period (12 slots=15 ms). Further, when the transmission path is varying fast, the control disables to follow the fast variation. Hence, the power control disadvantageously gives rise to a greater error than no control for the transmission power.
Further, this system utilizes a variable rate service. As mentioned above, this disadvantage is made more remarkable if the intermittent transmissions are executed. As the number of slots to be thinned out is increased, the control interval is made longer, so that the control cannot follow the far faster variation of the transmission path.
The interleave and the error correction are effective if the fading is too fast for the closed loop power control to follow. In the North America standard system that utilizes the CDMA, therefore, the combination or the closed loop power control, the interleave and the error-correcting codes is used for keeping the constant receiving quality how fast the fading is. The combination of the interleave and the error-correcting codes is effective in improving the quality of a subject channel, while the combination does not lead to avoidance of the increase of the interference to the other channels resulting from the increase of the average transmission power caused by the control error of the transmission power.
It is an object of the present invention to provide a mobile communication system for controlling transmission power which is arranged to utilize the CDMA (Code Divisional Multi Access) as in the foregoing North America standard system and reduce the interference of the subject channel given to the other channels by reducing the control error of the transmission power on the up link.
It is a further object of the present invention to provide a mobile communication system for controlling transmission power which is arranged to suppress increase of a control error for transmission power against fast fading and an oscillating phenomenon against very gradual fading, suppress increase of a control error for transmission power against intermittent transmissions for a variable rate, reduce the interference given by the subject channel to the other ones, increase the number of channels used for a frequency band at a time, and increase the number of accommodatable users as a system capacitance.